Field of the Invention
The present invention concerns a brake system intended for park braking a vehicle and constituted by a brake disk or other annular element forming a friction path, kinematically connected to at least one wheel of the vehicle and surrounded by a brake caliper which is actuated by a screw-nut assembly and which straddles the radial cross-section of the disk and applies to each of its faces a brake lining. The invention also concerns a disk brake caliper for this parking brake system.
The first brakes for wheeled vehicles, especially in the epoch of horse-drawn vehicles, were shoes applied through the intermediary of a braking bar on the treads of the wheels of an of the vehicle axle by means of a crank and screw system transforming by means of a nut the rotation of the screw or of the nut in axial application displacement. The screw which generally had a threaded portion with a square section head, was, however, irreversible by applying on axial force, thereby totally guaranteeing immobilization of the vehicle by maintaining at a halt the braking force applied by the driver via a relatively resilient transmission rigging. The resilient brake application stroke allowed not only modulation of the braking power by a more or less greater rotation of the screw, but furthermore, maintained this brake force at a halt even if slight variations in length of the brake elements occurred, for example following the cooling of the braking paths.
When the pressurized hydraulic brake was applied to more powerful vehicles, such as trains and automotive vehicles, the hydraulic brake which was well adapted for modulating the braking in meeting the needs of the driver, especially pressurized air in trains and heavy road vehicles, has never been able to guarantee the park braking of the vehicle over an extended period of time. The old hand parking brake fitted with a screw has thus remained in use for heavier vehicles such as locomotives and railway cars.
Over the last decades, in order to lighten the work of driving personnel and to remotely-control the park braking function, spring brakes have generally been used and have replaced the hand brakes even in railway applications. These spring brakes appear furthermore to ensure a safety function since they are generally released by pressurized air which is allowed to escape gradually in order to apply the graduated safety brake, the park braking being obtained through a total release of the pressurized air pressure. For railways, graduated service brakes have been produced which are directly controlled by springs with release of the pressurized air pressure in function of the desired braking force for the vehicle and of its load, so as to obtain the desired deceleration, whatever the load of the vehicle, the park or immobilization braking being automatically obtained, moreover in the case of total release or drain of the pressurized air pressure.
These spring brakes have greatly facilitated the driver's work since with their use it has been possible to control the application of the parking remotely through simple action upon a brake lever or an electric switch of the electric parking brake. On the other hand, the problems of accidental application of the parking brake, unknown during the times of the "hand applied" or park braking, have occurred upon sudden rupture of the pressurized hose for release of the spring brake. These accidental applications are generally aggravated by the fact that the driver of a heavy road vehicle then actuates the service brake through reflex, which leads in the case of heavy vehicles to the blocking of the wheels upon which the parking brake applies and to the vehicle leaving the road. For railway vehicles which are provided with a known service brake and parking brake, the train driver is generally not warned or warned too late that one of the parking brakes of his train has been accidentally applied and the train travels with the brake applied on the axle which has undergone an application of the spring brake thereby risking the destruction of the wheels of this axle.
In the case of powerful actuation of the service brake, the addition of the spring parking brake which persists for a long time prior to complete wear of the brake linings and of the service brake at maximal power, can lead to blocking the wheels of this disturbed axle and to the formation of a flattened portion of the wheels.